Double Fertilization

Fertilization in flowering plants or angiosperms is quite different and often called Double fertilization, a pivotal reproductive mechanism in flowering plants and a characteristic feature of angiosperms.

1.0Discoveries related to double fertilization

The actual fusion of the male and female gametes in fertilization in angiosperms is traced to a monographic publication by Strasburger (1884). This work was mostly devoted to the nuclear cytology of pollen grains and pollen tubes of plants belonging to a wide range of families and to the fate of male gametes delivered by pollen tubes in the embryo sacs of Orchis latifolia (Orchidaceae), and Monotropa Hypopitys (Monotropaceae). 

The breakthrough occurred when Nawaschin (1898) in Russia showed that in ovules of Lilium martagon and Fritillaria tenella (Liliaceae), both male gametes from the pollen tube penetrated the embryo sac; whereas one of them fused with the nucleus of the egg cell, the other fused with the polar fusion nucleus (at that time known as the definitive nucleus) floating in the central cell, initiating a second fertilization event. 

The phenomenon observed by Nawaschin was independently confirmed in L. martagon and L. pyrenaicum by Guignard in France. The account of this investigation was communicated to the Academy of Sciences in Paris on April 4, 1899, and was published soon afterwards in its Report (‘Comptes Rendus’) Guignard. 

2.0Definition of Double Fertilization

Double fertilization in flowering plants is a unique reproductive process. It involves the fusion of two male gametes from the pollen grain with two cells in the plant's female gametophyte (embryo sac).

Specifically, one male gametes fertilizes the egg cell, forming a zygote. This process is termed syngamy, and it is true fertilization. The zygote develops into the embryo, while the other male gametes combine with two polar nuclei to form a triploid cell. This process is termed triple fusion, which further develops into the endosperm. 

In an embryo sac, fusion occurs twice; one is syngamy, and another is triple fusion. Therefore, the phenomenon is known as double fertilization. Double fertilization was discovered by "Nawaschin" in Lilium and Fritillaria plants.

3.0Process of Double Fertilization

Double fertilization in angiosperms involves the fusion of two sperm cells from a pollen grain with two cells in the plant's female gametophyte (embryo sac). Here is a step-by-step explanation:

1. Pollination:

The process begins with the transfer of pollen grains from the male reproductive organ (anther) to the female reproductive organ (stigma) of the same or another flower. This can occur through various agents such as wind, water, insects, or animals.

2. Palynology:

The term palynology was first of all introduced by Hyde & Williams in 1944. The term palynology is derived from the Greek word “Palyenin” means to scatter or to sprinkle as many pollen grains or spores are easily dispersed or carried away. 

Palynology is a branch of science concerned with the study of spore and pollen study whether living or fossil. 

The term pollen was introduced by Swedish botanist Linnaeus in 1760 and it is derived from the Latin word “Pollinis” whose meaning is fine flour due to its granular nature. Pollen is a haploid male reproductive body produced as a result of meiosis in pollen mother cells containing male gametophytes of angiosperm & Gymnosperm. 

There are various branches of palynology- 

• Paleopalynology- concerned with the study of fossil spores and pollen. This branch is used to reconstruct past vegetation conditions.
• Geopalynolgy- pollen analytical investigation of quaternary deposits.
• Palynotaxonomy- Morphology of spores and pollen have a specific structure including the structure and ornamentation in case of spores and the number and position of aperture and ornamentation in case of pollen. The constant spore and pollen characters to the particular species are extremely helpful in the taxonomic identification of genus even at the species level.
• Aeropalynology- Concerned with the study of spores and pollen present in the air.
• Melittopalynology or Melissopalynolgy - study of pollen grains present in honey type A honey & type B - collected from two different places. 
• Iatropalynology- Concerned with the study of pollen grains and spores, which cause allergies like eczema, asthma, allergic lung disease, bronchitis, hay fever etc in susceptible humans.
• Pharmacopalynology- Study of pollen and spores present in drugs and tablets.
• Forensic palynology - helpful in criminology. It is used as an aid in crime detection.
• Copropalynology- Concerned with the study of pollen grains and spores present in external and waste products. 
• Biogenic palynology or pollen biology- It includes pollen stigma interaction, pollen viability/ germination, pollen storage, production of haploid plants and also in the plant breeding programs which may be utilized in hybridization.

3. Pollen tube formation:

Upon landing on the stigma, the pollen grain germinates, forming a pollen tube. The pollen tube grows down through the pistil's style (if present), guided by various chemical signals, to reach the ovary.

4. Ovule and embryo sac development:

5. Entry of pollen tube into the ovule: The pollen tube penetrates the micropyle, a small pore in the ovule's integuments, and enters the embryo sac (female gametophyte). This structure contains the female gametes (egg cell and two polar nuclei) surrounded by supportive cells.

6. Double fertilization:

(a) The first male gamete fuses with the egg cell (syngamy), forming a diploid (2n) zygote. This zygote will develop into the embryo of the seed.

(b) Simultaneously, the second male gamete fuses with two polar nuclei (triple fusion), forming a triploid cell called the primary endosperm nucleus (PEN). This triploid (3n) cell develops into the endosperm, a tissue that provides nourishment to the developing embryo.

7. Embryo and endosperm development: The zygote undergoes mitotic divisions and develops into the embryo, while the primary endosperm nucleus undergoes further divisions and develops into the endosperm tissue. The endosperm serves as a nutritive tissue for the embryo, providing essential nutrients for its growth and development.

(a) Seed formation: The mature ovule develops into a seed, containing the embryo and the endosperm, encased within protective seed coats derived from the ovule's integuments.

(b) Development of Embryo in Angiosperms ( Embryogenesis): 

After fertilization, a series of changes occur in the ovule, and finally, the seed is formed. Side by side with the development of the endosperm, the zygote develops into the embryo after a period of rest. The development process of a mature embryo from a zygote is called embryogenesis or embryogeny. The embryo has the potential to develop into a complete plant.

In all flowering plants, embryogenesis begins with the division of the zygote. The zygote usually divides to form a two-celled proembryo by creating a wall across. One cell near the micropyle is called the basal cell, while the smaller cell facing toward the center of the embryo sac is termed the terminal or apical cell. Initially, there are no major differences in early developmental stages between monocotyledons and dicotyledons, and development proceeds similarly until the globular stage. Variations emerge when the initial formations of the plumule and cotyledons occur.

Dicot Embryo: 

• The embryo forms at the micropylar end of the embryo sac, where the zygote is situated. In many cases, the zygote only divides after a sufficient amount of endosperm has been produced. This delay in division ensures that the developing embryo has a reliable source of nutrition.
• The first division of the oospore is transverse, resulting in the formation of two cells. One cell, known as the basal or suspensor cell, lies towards the micropyle, while the other cell, termed the apical or terminal or embryonal cell, lies towards the chalaza. The basal cell (suspensor cell) and embryonal cell divide simultaneously.
• The embryonal cell undergoes mitotic divisions to generate the proembryo, followed by the development into the globular, heart-shaped, and mature embryo. Meanwhile, the suspensor cell undergoes transverse divisions, forming a filament-like structure (6 to 10 celled) called the suspensor. The main role of the suspensor is to push the developing embryo into the nutrient-rich endosperm to facilitate nutrition uptake.
• The micropylar cell of the suspensor enlarges, becoming known as the haustorial cell. In Capsella, the embryo's curved position is due to the ovule's curved body, resulting in what is termed a "torpedo" or mature embryo.
• A typical dicot embryo comprises an embryonal axis and two cotyledons. The embryonal axis, also known as the tigellum or main embryonal axis, extends between the plumule and radicle. The portion of the embryonal axis above the cotyledon level is called the epicotyl, which ends with the plumule or stem tip. Below the cotyledon level lies the hypocotyl, terminating in the radicle or root tip, covered by the root cap.
• Both cotyledons are positioned laterally along the embryonal axis, while the plumule forms at the terminal end in dicotyledon embryos.

Monocot Embryo: 

• The zygote or oospore elongates and then divide trans­versely to generate basal and terminal cells, which are the final stages of development. This cell is produced by the basal cell (which is located toward the micropylar end) and is big, thick, and swollen. It has the potential to serve as a haustorium. 
• This is single cell suspensor in monocots. The terminal cell divides by a second transverse wall, resulting in the formation of two cells. 
• Monocotyledonous embryos possess only one cotyledon. In mature embryos of wheat, a member of the grass family (Poaceae), this single cotyledon is known as the scutellum. Positioned laterally along the embryonal axis, the scutellum is distinct. 
• Below the scutellum lies the radicle, with its root cap enclosed within a protective sheath called the coleorhiza.
• Above the level of the scutellum is the epicotyl, housing the shoot apex and a few leaf primordia enveloped within a hollow foliar structure called the coleoptile. 
• Adjacent to the coleorhiza, there is a small outgrowth known as the epiblast, which represents the beginnings of a second cotyledon.

4.0Embryogeny in Monocotyledons

Proembryo stage → Globular stage → Scutellar stage → Coleoptilar stage